While there is no doubt that "£35 is a lot to some poor student", how would that explain anything? One way or another, the putative poor student is going to spend that £35. (Or even more, in your scenario of "perhaps buying another item".)

Wanting to pay by check is of course a possibility, but would that really be a strong enough motive to risk losing _this particular item_ by delaying to propose a reserve? We're not talking about just any old book here. How many ABE customers don't have credit cards -- particularly among those who would be watching for _this_ item?